With the progress of communicating technique, electronic products such as cell phones, computers and even house electric equipment are getting to have intelligent, portable and mobile functionalities, so that efficiency of exchanging information between users and such electronic products become a critical point of the progress of communicating technique. For an aspect of clearly and efficiently delivering information to users with such electronic products, a display of such electronic products is designed to meet multiple demands such as high efficiency, upgraded quality, large memory capacity, lower weight, lower cost and lower power consumption. Consequentially, the conventional CRT display was replaced by the liquid crystal display (LCD) in just a few years.
In early stage, the LCD is the twisted nematic (TN) or the super twisted nematic (STN) type, and liquid crystal molecules with a chiral agent disposed in the TN/STN type LCD is a positive (nematic) type. A long axis of the positive type liquid crystal molecules is parallel to a surface of each substrate of the TN/STN type LCD when no electricity is applied thereto. An orientation of the positive type liquid crystal molecules is determined by a rubbing direction (i.e. alignment direction) formed on an alignment layer which is usually made of polyimide and disposed on the surface of each substrate of the TN/STN type LCD. The alignment directions of the alignment layers respectively disposed on the two substrates of the TN/STN type LCD are perpendicular to each other, whereby the positive type liquid crystal molecules exhibit a continuously twisted arrangement from one substrate to the other substrate, wherein a twisted angle of the positive type liquid crystal molecules in the TN type LCD is about 90 degree, and a twisted angle thereof in the STN type LCD is about 270 degree. Besides the two substrates and the positive type liquid crystal molecules disposed therein, the TN/STN type LCD includes two polarizer layers and a backlight, wherein the two polarizer layers are respectively adhered to outer surfaces of the two substrates and perpendicular to a light-absorbing axis of each substrate. Light emitted from the backlight is polarized by one polarizer layer disposed between the substrate and the back light, then a polarized direction of the polarized light is transformed by the positive type liquid crystal molecules arranged at the twisted angle, and then the polarized light passes through the other polarizer layer, so as the TN/STN type LCD is on transmitting state (also called “white state”). When a voltage is applied to the TN/STN type LCD, the long axis of the positive type liquid crystal molecule tends to align according to the direction of electric field, and the polarized light is passed through the untwisted liquid crystal molecules then reflected by the other polarizer layer, so as the TN/STN type LCD is on reflecting state (also called “black state”). Drawbacks of the TN/STN type LCD are small viewable angle, severe brightness difference and color difference at large view angles. Therefore, a compensation film must be applied thereto for correcting such drawbacks, and it causes to increase the manufacture cost of the TN/STN type LCD.
A thin-film transistor liquid crystal display (TFT-LCD) of Multi-domain vertical alignment (MVA) type provides an excellent solution to the view angle limitation that the TN/STN type LCD is subjected to, and it uses negative (nematic) type liquid crystal molecules and vertical type alignment films. When no voltage is applied to the MVA type TFT-LCD, the long axis of the negative type liquid crystal molecules is perpendicular to the surface of the substrate. A voltage applied to the MVA type TFT-LCD would cause the negative type liquid crystal molecules to tilt, and the long axis of the negative type liquid crystal molecules is allowed to align in a direction perpendicular to the electric field. To overcome the view angle limitation, a pixel of the MVA type TFT-LCD is divided into multiple domains, and the negative type liquid crystal molecules disposed therein are caused to tilt in different directions so that the MVA type TFT-LCD can provide similar viewing effect at various directions.
Several ways can be adopted to allow liquid crystal molecules, disposed in different domains of a pixel, orienting in different directions respectively. As shown in FIG. 1, the first way is to form bumps 5 on upper and lower substrate 1, 2 having ITO electrodes 3, 4 respectively formed thereon by means of exposing development, so that the bumps 5 can cause a pre-tilt angle for liquid crystal molecules around the bumps 5, and thus guide the other liquid crystal molecules 6 to tilt to predetermined directions.
As shown in FIG. 2, the second way is to form upper and lower ITO electrodes 22, 24 that are of predetermined patterns on upper and lower substrates 12, 14 respectively, so that an electric field induces a predetermined tilt angle, thereby controlling the orientation of the liquid crystal molecules 60 in different domains. The second way is often called patterned vertical alignment (PVA).
As shown in FIG. 3, the third way, so-called polymer stabilized vertical alignment (PSVA) technique, is to form a patterned ITO electrode 204, usually formed as a fishbone type, on one substrate 104, and to form a full-layer ITO electrode 202 (Full ITO) over the other substrate 102, and polymerizable monomers 800 are added in the liquid crystal medium. An electric field is firstly applied to cause tilting liquid crystal molecules 600 in the liquid crystal medium, and then irradiation of ultraviolet is applied to cause a polymerization of the polymerizable monomers 800 to form bumps 400 deposited on the surface of the substrate, and the bumps 400 can guide a tilting angle of the liquid crystal molecules 600. In comparison to the other MVA techniques, the PSVA technique has a lot of advantages such as higher transmission, higher contrast and faster response, so that the PSVA technique becomes a mainstream technique of fabricating large-size TFT-LCD. A key point in the PSVA technique is to control a polymerization of the polymerizable monomers, wherein to control the polymerization reaction includes a photo-reaction rate, homogeneity of the bumps, and unreacted residue of the polymerizable monomers. A high quality PSVA LCD may be obtained after the above key point has been well controlled. However, initiation efficiency of the polymerization thereof is relatively low by using conventional photo-initiators.
Therefore, there is a need of providing a novel photo-initiator which can effectively utilize energy of ultraviolet so as to increase initiation efficiency of the polymerization thereof.